Ultra-Lightweight Kiln Cars

A new kiln car base system offers all of the advantages of both ceramic fiber and dense refractory bases, with none of their disadvantages.

It has long been recognized that low-thermal-mass kiln cars can provide ceramic manufacturers with greater control of their firing schedules and significantly reduced process fuel costs. Refractory ceramic fiber can provide one of the most cost-effective components for such cars, but unprotected fiber presents unacceptable process and environmental risks.

Recently, however, a new kiln car base system was introduced that overcomes these challenges. The new system uses an encapsulating technology that enables it to provide the unrivalled benefits of ceramic fiber without the environmental and product-quality costs historically associated with its use.

Figure 1. Early kiln cars used dense, heavy refractories that typically weighed 10-20 times more than the product being fired.

The Move from Dense to Lightweight Cars

Kiln designers of the 1960s and earlier thought that kiln cars should be constructed from the highest refractory grades used in the kiln chamber, and that they should also exhibit load-bearing properties. As a result, only relatively dense, heavy refractories, such as bricks and refractory concretes, were used. In many cases, the kiln car refractories alone typically weighed 10-20 times more than the product being fired (see Figure 1).

Users soon began to realize that these heavy kiln car constructions greatly influenced their overall manufacturing programs and production schedules. The high thermal capacity of heavy cars made it almost impossible to maximize kiln capacity by reducing cycle times, or to follow the preferred heating and cooling temperature profiles that would avoid introducing thermal shock to the products (dunting). Research and development programs showed that body formulations, particularly those used in the whitewares sector, could be successfully fired much more quickly than was then the norm, but the heavy kiln cars would not allow cycle times to be shortened, thus denying to manufacturers an important means of reducing manufacturing costs. Where plant output was being limited by kiln capacity, any increase required new kilns to be installed at a high capital cost.

Figure 2. Open ceramic fiber kiln cars, introduced in the early ’70s, were lightweight but created problems with kiln dirt and health issues.

In the early 1970s, ceramic fiber was introduced as a layer of blanket laid on the upper surface of dense kiln cars as a thermal barrier. Basic trials quickly confirmed the material’s excellent insulating properties. Ceramic fiber in many shapes, forms and methods of installation became accepted by the industry as it allowed ceramic manufacturers to speed up their firing cycles. Kilns were redesigned to operate on schedules closer to the capabilities of the materials and products being fired—in some cases, for example, firing in 12-hour cycles for mixed sanitaryware items resulted in a 50% output increase over the old high-thermal-mass kilns of the same size. Open ceramic fiber lightweight kiln cars, such as the one shown in Figure 2, were a major factor in this development.

In addition to allowing much faster firing cycles, the lightweight cars also provided other advantages, including:

lower specific energy consumption from the shorter cycle times usually possible,

reduced bearing maintenance from lower undercar temperatures,

reduced bearing and pusher wear from lower car weights, and

no need for post-kiln cooling due to the low thermal mass.

Problems Associated with “Open-Fiber” Cars

Unfortunately, the new car constructions were not without their own problems. After only a few years of using the new cars, manufacturers began to notice that ware losses due to “kiln dirt” increased significantly. Investigations revealed that the higher velocity combustion systems necessary to achieve the faster firing cycles were propelling the fibers on the kiln car surfaces airborne. This resulted in some erosion of the kiln car deck, which, in combination with the more severe thermal cycling of the kiln cars on a faster turnaround, exacerbated what was now becoming a major cause of firing losses.

It was not uncommon for a 1980s sanitaryware kiln to have a fleet of kiln cars in excess of 200, all of open-fiber design. Manufacturers using open-fiber kiln cars faced the options of:

replacing their open-fiber kiln car linings on a regular basis and introducing a maintenance cost not planned for at the time of the kiln purchase,

continuing to use the kiln and seeing otherwise good products downgrade in quality due to fiber dirt, or

going back to dense refractories and reduced output levels.

The dilemma was further aggravated by the fact that the existing fiber, although causing losses, still retained its insulating properties. This meant that the kiln dirt problem would have to bear the full justification for fiber replacement. Going back to slower cycle times would require additional kilns, which meant investing in additional buildings and so was usually cost-prohibitive

A further serious consideration emerged during the 1980s and early 1990s, when health and safety concerns with respect to the use of refractory ceramic fibers became a growing issue. At this point, the only option became encapsulating or even eliminating the fiber.

Figure 3. The new ultra-lightweight kiln car peripheral block system uses just two elements to produce a totally encapsulated, lightweight, robust and durable kiln car base.

Lightweight Alternatives to Open-Fiber Cars

The 1990s saw the introduction of a range of relatively lightweight systems designed to overcome or avoid the problems of open-fiber cars. Systems based on insulating brick formulations successfully tackled all of the disadvantages associated with fiber, but they could not match fiber’s extremely low thermal mass and highly insulating properties. The most thermally acceptable solutions were still those based on ceramic fiber, so some companies began developing ways to totally encapsulate the fiber in hard refractory tiles and/or insulating brick.

Through wide market research completed in the late 1980s and early 1990s, and after listening to the views of ceramic manufacturers—particularly sanitaryware and tableware manufacturers—a group of researchers from Dyson Hotwork determined that the ideal solution to the dilemma presented by ceramic fiber would need to recognize that:

Kiln dirt can be traced back to kiln car refractories, and fiber degradation occurs on a progressive basis and even affects comparatively new installations.

Based on these observations, a new ultra-lightweight kiln car peripheral block system* was introduced in the late 1990s that comprises only two elements to produce a totally encapsulated, lightweight, robust and durable kiln car base (see Figures 3 and 4).

The Finished Product

The original slip-cast product featured a poor cosmetic finish and was somewhat friable, but over the past two years and in conjunction with DFC Thermal Ceramics, the ultra-lightweight kiln car systems began being manufactured in a new body and through a new process, yielding a strong, clean-lined, well-finished product. Promising trials have been followed by successful installations in sanitaryware and tableware kiln cars of both tunnel and intermittent operation.

The ultra-lightweight kiln car systems can be individually tailored for most car types. When used in conjunction with low-thermal-mass kiln furniture, the cars can reduce overall car weight by up to 65%, depending on the original structure. Such significant improvements in the ware-to-car-weight ratio can offer kiln users the potential to speed up kiln cycle times or push rates (where the ware body allows), or to take advantage of the reduced energy consumption associated with ultra-low thermal mass kiln cars.

The ultra-lightweight kiln car systems can normally be installed on a conventional car within one day, and any necessary future repairs can be carried out in minutes. Depending on the kiln car’s design, the car might not even need to be removed from service during repair. This can minimize the fleet size since only a minimal number of spare cars will be required.

Figure 5. In the first stage of a new installation, the support props are put into place. The base can be designed for a recrystallized silicon carbide beam application, as shown here, or for a multiprop system.

Installation

With their unique, simple and effective design and method of manufacture, the ultra-lightweight kiln car systems can be customized and configured for both new kiln installations and for retrofits to existing kiln car fleets.

Figure 6. In the second stage of a new installation, a leveling layer of either ceramic fiber or fiber board is added, followed by the positioning of the blocks.

Retrofit Applications

To minimize disruption in kiln operation and avoid a single large capital commitment when retrofitting the system, the new cars must be both dimensionally and thermally compatible with the old.

When retrofitting open fiber cars, only minimal interference to existing linings is necessary. In this case, the outer perimeter of the kiln car is removed and is replaced with the ultra-lightweight kiln car system, and the cover batts are then added. Approximately 80% of the existing lining remains in position and undisturbed. In the event that the existing open (normally stack-bonded) fiber is of inadequate density to support the cover batts, the fiber can be packed out with fiber mastic and/or extra fiber, or support tiles can be inserted vertically to offer the necessary support.

When converting from an existing dense kiln car structure, the weight can potentially be reduced by up to 70%, but the entire upper section of the base will need to be rebuilt. Additionally, the thermal characteristics of the new car bases will be very different from the existing bases. The much lower thermal demand of the new cars will result in more rapid heating of the ware, with potentially damaging effects on product quality due to the inability of multi-car kilns to control down to a one-car zone length.

This problem can usually be minimized or even eliminated by the one-off introduction of a highly insulating layer immediately above the dense base of every car. This will produce a “wave front” for the first few such cars, after which the kiln will settle down with the temporarily modified fleet. The actual material temporarily used will depend the particular kiln application, but the principle involved is that the new retrofitted ultra-lightweight cars will create a much-reduced disturbance to the environment experienced by the ware as they are slowly introduced over the months involved.

Figure 7. The third stage of a new installation. Following the final placement of the blocks and installation of the preformed fiber block infill, the cover batts are placed, and the car is ready for the superstructure.

New Installations

The ultra-lightweight kiln car systems are ideally suited to new kiln installations. Figures 5-7 illustrate the main stages of a new installation. Installation times generally vary between 30-90 minutes, depending on the car size.

The Best of Both Worlds

The new ultra-lightweight kiln car block system offers all the advantages of both ceramic fiber and dense refractory bases, with none of their disadvantages. It can be installed new or retrofitted over a long period to render the capital implications more palatable. Because of the new kiln car block system’s benefits, it will undoubtedly provide the basis for manufacturers to extract maximum value from their kilns without posing risks to their employees or their product quality.